So for a road car with an MG Metro 12G940 head slightly modded, rebore to 1293, Evo001 Cam, Should I have the stock Cast Iron Flywheel lightened.
The biggest argument for lightening a flywheel is that for ever after, you drive round with 3 lbs less weight, but that will make a sod all point 02% difference to your acceleration.
The math formula for spinning up the flywheel and accelerating the car is the same, and it contains the element Time. Mass X Meters per Second per Second.
Some people, claiming that a lighter flywheel requires less energy to spin it up to a given speed, assume that that energy would be available to accelerate the car, which all things being equal, is correct.
However, things are not equal as it takes much more time, and therefore less energy per second, to spin the flywheel up to the given speed with the added load of accelerating the car, than it does to spin up an unloaded flywheel.
Put simplistically, to make things equal, the weight of the flywheel would have to be reduced to its square root for the assumed energy to become available, because in the acceleration formula the Time element is squared,